The fuel economy of a car, measured in the miles per gallon, is modeled by the function f(s)= -0.0009s^2+ 0.699s+12 where s represents the average speed of the car, measured in miles per hour. what average speed results in the maximum fuel economy? a. -38.83 miles per hour b. 38.83 miles per hour c. 3.88 miles per hour d. 25.57 miles per hour